A series of studies designed to identify factors influencing the incidence of stroke in the stroke-prone rat strain are in progress. A natural ingredient diet manufactured in accordance with a formulation obtained from Japan and reported to produce a high incidence of stroke in this rat strain, as well as purified diets varying in casein and salt are being evaluated. Preliminary analyses of the data indicate a high percentage of the stroke-prone rats die before they are 15 months old irrespective of diet. However, the primary cause of death was congestive heart failure rather than stroke. The diet made in accordance with the Japanese formulation had no effect on the death rate of the rats. Rats fed diets containing 10 or 30% casein died sooner than those fed a 20% concentration and rats fed 5 or 2.5% dietary salt died sooner than those fed 0.25%.